1 Chapter 14 Carbohydrates 13.1 Carbohydrates

2 Carbohydrates are a major source of energy from our diet. composed of the elements C, H, and O. also called saccharides, which means “sugars.”

3 Carbohydrates are produced by photosynthesis in plants. such as glucose are synthesized in plants from CO 2, H 2 O, and energy from the sun. are oxidized in living cells to produce CO 2, H 2 O, and energy.

4 Types of Carbohydrates The types of carbohydrates are monosaccharides, the simplest carbohydrates. disaccharides, which consist of two monosaccharides. polysaccharides, which contain many monosaccharides.

5 Monosaccharides Monosaccharides consist of 3-6 carbon atoms typically. a carbonyl group (aldehyde or ketone). several hydroxyl groups.

6 Aldoses Aldoses are monosaccharides with an aldehyde group. many hydroxyl (-OH) groups. triose (3 C atoms) tetrose (4 C atoms) pentose (5 C atoms) hexose (6 C atoms) aldose Erythose, an aldotetrose

7 Ketoses Ketoses are monosaccharides with a ketone group. many hydroxyl (-OH) groups. CH 2 OH │ C=O ketose │ H─ C─OH │ H─ C─OH │ H─C─OH │ CH 2 OH Fructose, a ketohexose

8 Chapter 14 Carbohydrates 13.2 Fischer Projections of Monosaccharides

9 Fischer Projections A Fischer projection is used to represent carbohydrates. places the most oxidized group at the top. shows chiral carbons as the intersection of vertical and horizontal lines. Emil Fischer

10 D and L Notations In a Fischer projection, the −OH group on the chiral carbon farthest from the carbonyl group determines an L or D isomer. left is assigned the letter L for the L -form. right is assigned the letter D for the D -form.

11 Examples of D and L Isomers of Monosaccharides D -Glucose D -Ribose L -Galactose

12 Learning Check Identify each as the D or L isomer. A.B. C. __-Ribose __- Threose __- Fructose

13 Solution Identify each as the D or L isomer. A.B. C. L -Ribose L -Threose D -Fructose

14 D -Glucose D -glucose is found in fruits, corn syrup, and honey. an aldohexose with the formula C 6 H 12 O 6. known as blood sugar in the body. the monosaccharide in polymers of starch, cellulose, and glycogen.

15 Blood Glucose Level In the body, glucose has a normal blood level of mg/dL. a glucose tolerance test measures blood glucose for several hours after ingesting glucose.

16 D -Fructose is a ketohexose, C 6 H 12 O 6. is the sweetest carbohydrate. is found in fruit juices and honey. converts to glucose in the body.

17 D -Galactose is an aldohexose, C 6 H 12 O 6. is not found free in nature. is obtained from lactose, a disaccharide. has a similar structure to glucose except for the –OH on carbon 4.

18 Chapter 14 Carbohydrates 13.3 Haworth Structures of Monosaccharides Sir (Walter) Norman Haworth

19 Cyclic Structures Cyclic structures are the prevalent form of monosaccharides with 5 or 6 carbon atoms. form when the hydroxyl group on carbon 5 reacts with the aldehyde group or ketone group.

20 Drawing the Cyclic Structure for Glucose STEP 1: Number the carbon chain and turn clockwise to form a linear open chain

21 Cyclic Structure for Glucose STEP 2: Fold clockwise to make a hexagon. Bond the carbon 5 –O– to carbon 1. Place the carbon 6 group above the ring. Write the –OH groups on carbon 2 and carbon 4 below the ring. Write the –OH group on carbon 3 above the ring. Write a new –OH on carbon

O CH 2 OH OH 22 Cyclic Structure for Glucose (continued)  - D -Glucose  - D -Glucose  STEP 3: Write the new –OH on carbon 1 down for the  form. up for the  form. 

23 Example of the Formation of Cyclic Glucose

O CH 2 OH OH 24  - D -Glucose and β- D -Glucose in Solution When placed in solution, cyclic structures open and close.  - D -glucose converts to β- D -glucose and vice versa. at any time, only a small amount of open chain forms.  - D -Glucose D -Glucose (open) β- D -Glucose (36%) (trace) (64%) O H CH 2 OH OH O C H OH OH OH OH OH OH CH 2 OH O

25 Cyclic Structure of Fructose Fructose is a ketohexose. forms a cyclic structure. reacts the —OH on carbon 5 with the C=O on carbon 2. D -Fructose  - D -Fructose α- D -Fructose

26 Chapter 13 Carbohydrates 13.4 Chemical Properties of Monosaccharides

27 Reducing Sugars Reducing sugars are monosaccharides that oxidize to give a carboxylic acid. undergo reaction in the Benedict’s test. include the monosaccharides glucose, galactose, and fructose.

28 Oxidation of D -Glucose [O]

29 Reduction of Monosaccharides The reduction of monosaccharides involves the carbonyl group. produces sugar alcohols, or alditols. such as D -glucose gives D -glucitol, also called sorbitol.

30 Learning Check Write the products of the oxidation and reduction of D -mannose. D -Mannose

31 Solution Write the products of the oxidation and reduction of D -mannose. D -Mannitol D -Mannose D -Mannonic acid

32 Chapter 14 Carbohydrates 13.5 Disaccharides

33 Important Disaccharides A disaccharide consists of two monosaccharides. Monosaccharides Disaccharide Glucose + glucose maltose + H 2 O Glucose + galactoselactose + H 2 O Glucose + fructosesucrose + H 2 O

34 Maltose Maltose is a disaccharide also known as malt sugar. composed of two D -glucose molecules. obtained from the hydrolysis of starch. linked by an  -1,4-glycosidic bond formed from the  − OH on carbon 1 of the first glucose and − OH on carbon 4 of the second glucose. used in cereals, candies, and brewing. found in both the  - and β -forms.

35 Formation of Maltose α-Form

36 Lactose is a disaccharide of β- D -galactose and α- or β- D -glucose. contains a β -1,4- glycosidic bond. is found in milk and milk products.

37 Sucrose Sucrose or table sugar is obtained from sugar cane and sugar beets. consists of α- D -glucose and β- D -fructose. has an α,β-1,2-glycosidic bond.

38 Sweetness of Sweeteners Sugars and artificial sweeteners differ in sweetness. are compared to sucrose (table sugar), which is assigned a value of 100.

Discovered in 1976

Discovered in 1879

Discovered in 1967

Discovered in 1965

Agave nectar (sometimes called agave syrup) is most often produced from the Blue Agaves that thrive in the volcanic soils of Southern Mexico. Agaves are large, spikey plants that resemble cactus or yuccas in both form and habitat, but they are actually succulents similar to the familiar Aloe Vera. To make the agave nectar, sap is extracted from the pina, filtered, and heated at a low temperature, which breaks down the carbohydrates into sugars. Lighter and darker varieties of agave nectar are made from the same plants. Because of the low temperatures used in processing many varieties (under 118°F) raw foods enthusiasts generally regard agave nectar as a raw food. The taste of agave nectar is comparable, though not identical, to honey. Many people who do not like the taste of honey find agave a more palatable choice. It also has none of the bitter aftertaste associated with artificial sweeteners.

Stevia (sweetleaf, sweet leaf or sugarleaf) This sweetener is made from a crude preparation (powder or liquid) of dried stevia leaves. It may contain a mixture of many substances, only some of which are sweet.

Truvia™ natural sweetener is made from rebiana, the best tasting part of the stevia leaf, erythritol and natural flavors. Rebiana is the common or usual name for a food-grade high-purity extract of the stevia leaf that is at least 97 percent rebaudioside-A, the best tasting sweet substance found in the stevia leaf. Chemically, erythritol is simply a four-carbon sugar alcohol. Erythritol is made by fermenting glucose then separating and purifying the resulting product.

Most fruits, berries and plants contain xylitol (also called wood sugar), the richest natural sources being plums, strawberries, raspberries, cauliflower and endives.

Xylitol is extremely toxic to dogs The toxic dose of xylitol is 0.1 gm/kg body weight, while liver failure results from doses greater than 0.5 g/kg body weight. Translating these numbers into something usable in the every-day world is a little harder to do, since the amount of xylitol varies from one product to another. Two sticks of gum is enough to cause a serious drop in blood sugar for a small (under 20 lb) dog, while it might take 8 to 10 sticks to affect a large (over 60 lb) dog, but these amounts are only an estimate. As for baked goods containing xylitol, again, the amount in each cookie or muffin will vary. In one case, a Standard Poodle died after eating 5 or 6 cookies sweetened with xylitol.

48 The history of sodium cyclamate illustrates the difficulty in balancing consumer safety with the needs of the consumer market. This sweetener was banned in 1970 because of research that indicated risks of cancer from consuming the sweetener. Discovered in 1937

49 Chapter 14 Carbohydrates 13.6 Polysaccharides

50 Polysaccharides are polymers of D -glucose. include amylose and amylopectin, starches made of α- D -glucose. include glycogen (animal starch in muscle), which is made of α- D - glucose. include cellulose (plants and wood), which is made of β- D -glucose. α- D -Glucose

51 Structures of Amylose and Amylopectin

53 Amylose Amylose is a polymer of α- D - glucose molecules. linked by  -1,4 glycosidic bonds. a continuous (unbranched) chain.

54 Amylopectin is a polymer of α- D - glucose molecules. is a branched-chain polysaccharide. has α-1,4-glycosidic bonds between the glucose units. has α-1,6 bonds to branches.

55 Glycogen is the polysaccharide that stores α- D -glucose in muscle. is similar to amylopectin, but is more highly branched.

56 Cellulose is a polysaccharide of glucose units in unbranched chains. has β-1,4-glycosidic bonds. cannot be digested by humans because humans cannot break down β-1,4- glycosidic bonds.